In 1940, Soviet secret police, under orders from Josef Stalin’s Politburo, executed more than 20,000 Polish prisoners of war captured during the Soviet invasion of Poland. For decades afterward, the events of the Katyn massacre were heavily debated and a source of tension between Russians who sought to preserve the WWII-era Soviet Union’s reputation, and Polish who sought the truth and an acknowledgement of wrongdoing.

According to Robert Smigielski of the Polish Institute of International Affairs in Warsaw, “The anniversary of the crimes at Katyn are very important for the Polish people. ... You can say that [the Katyn Massacre] is the most problematic issue in relations between the Poles and the Russians,” The New York Times reports.

The Nazi regime publicized the massacre as a way to undermine the Soviet Union’s relations with its Western allies and the Polish government-in-exile.

The Soviets launched their own investigation and determined that the Nazis had carried out the massacre in 1941. The cover-up was aided by Britain and the United States, who kept news of the massacre silent so as not to irritate their Soviet ally.

The U.S. government tried to keep news of the massacre silent, in an attempt to maintain good relations with the Soviet Union. Rutgers professor Adam Scrupski believes that historians must now take a closer look at the American role in the cover-up.

The Stalin Society, formed in 1991 to defend Stalin’s reputation, still asserts that the massacre was carried out by the Nazis. It argues that the story was fabricated by Nazi propagandists as a way to undermine Soviet diplomatic relations.

The United States’ attitude toward Katyn is illustrated in a collection of Time magazine covers from 1943, 1944, 1952 and 1972. The 1943 article suspects that Goebbels’s account is a piece of propaganda, while the 1944 article blames the Nazis. By 1952, the magazine placed the blame on the Soviets.